In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units (UE) such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB” or “B node”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations.
In some versions (particularly earlier versions) of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network providing wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In some WCDMA networks, as many as a few hundred radio base station nodes may be connected to an RNC. In WCDMA, a radio network controller (RNC) and its subservient radio base stations are collectively known as a Radio Network System (RNS). Similarly, in GSM the radio base stations (typically called “base transceiver stations” or “BTS” in GSM) are connect to a base station controller (BSC) node, with a BSC and its subservient base transceiver stations being collectively referred to as a Base Station Subsystem (BSS).
Evolved UTRAN (E-UTRAN), also referred to as Long Term Evolution (LTE), is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. FIG. 1 shows high level architecture for Long Term Evolution (LTE).
As shown in FIG. 1, E-UTRAN employs a mobility management entity (MME) as a control node. The MME is responsible for idle mode UE tracking and paging procedure including retransmissions. The MME is involved in the bearer activation/deactivation process and is also responsible for choosing the serving gateway (SGW) for a user equipment unit (UE) at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. The MME is responsible for authenticating the user.
In general, in E-UTRAN the functions of a radio network controller (RNC) node are performed by the radio base stations nodes. As such, the radio access network (RAN) has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes. The evolved UTRAN thus comprises evolved base station nodes, e.g., evolved NodeBs or eNBs, providing evolved UTRA user-plane and control-plane protocol terminations toward the user equipment unit (UE).
FIG. 2 shows the control plane stack for 3GPP LTE architecture; FIG. 3 shows the user plane stack for 3GPP LTE architecture. Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. In the control-plane, the NAS protocol, which runs between the MME and the UE, is used for control-purposes such as network attach, authentication, setting up of bearers, and mobility management. All NAS messages are ciphered and integrity protected by the MME and UE. The RRC layer in the eNB makes handover decisions based on neighbor cell measurements sent by the UE, pages for the UEs over the air, broadcasts system information, controls UE measurement reporting such as the periodicity of Channel Quality Information (CQI) reports and allocates cell-level temporary identifiers to active UEs. It also executes transfer of UE context from the source eNB to the target eNB during handover, and does integrity protection of RRC messages. The RRC layer is responsible for the setting up and maintenance of radio bearers. The radio link control (RLC) layer is the layer used to format and transport traffic between the eNodeB and the user equipment unit (UE).
The eNB hosts the following functions (among other functions not listed): (1) functions for radio resource management (e.g., radio bearer control, radio admission control), connection mobility control, dynamic resource allocation (scheduling); (2) mobility management entity (MME) including, e.g., distribution of paging message to the eNBs; and (3) User Plane Entity (UPE), including IP Header Compression and encryption of user data streams; termination of U-plane packets for paging reasons, and switching of U-plane for support of UE mobility. The eNB hosts the physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. The eNodeB also offers Radio Resource Control (RRC) functionality corresponding to the control plane. The eNodeB performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.
The eNBs are connected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to the Evolved Packet Core (EPC). The S1 interface supports a many-to-many relation between an access gateway (aGW) in the packet core and the eNBs. The S1 interface provides access to the Evolved RAN radio resources for the transport of user plane and control plane traffic. The S1 reference point enables MME and UPE separation and also deployments of a combined MME and UPE solution.
There are two levels of re-transmissions for providing reliability for LTE, namely, the Hybrid Automatic Repeat reQuest (HARQ) at the MAC layer and outer ARQ at the RLC layer. The outer ARQ is required to handle residual errors that are not corrected by HARQ that is kept simple by the use of a single bit error-feedback mechanism.
As in pre-LTE WCDMA networks, at some point it may be determined that a user equipment unit (UE) has relocated or moved so that it would now be better served by a new base station (“target” base station) rather than the base station which previously served the user equipment unit (the “source” base station). Such determination can be made, for example, upon comparison of signal strength measurements or the like received from the source base station and the target base station. Upon making such a determination, the user equipment unit (UE) participates with the radio access network in a “handover” process whereby traffic involving the user equipment unit (UE) is rerouted to the target base station.
A procedure for an intra-LTE hand-over is shown in FIG. 4. In order to avoid packet losses during hand-over, the procedure involves the transfer of buffered data from the source eNodeB to the target eNodeB. This transfer is started after step 4-4 (e.g., after the hand-over command).
It has been agreed in LTE that, in conjunction with the handover, a radio link control (RLC) protocol context will not be transferred, but rather the complete RLC SDUs that have not been received by the user equipment unit (UE) will be transmitted from the source to the target eNodeB.
In E-UTRAN, the target base station can be referred to as a “target eNodeB” and the source base station can be referred to as a “source eNodeB”. In the handover procedure for LTE, knowing exactly which packets of the traffic stream destined to the user equipment unit (UE) need to be rerouted to the target eNodeB (those not already successfully received by the user equipment unit (UE)), and knowing when to perform the handover, can be problematic. Part of the problem resides in the fact that, while the eNodeB sends packets to the user equipment unit (UE), those packets are received by the eNodeB in the form of a larger unit as part of the radio link control (RLC) protocol, the larger unit typically being called a service data unit (SDU).
There are at least two possible ways to determine which Service Data Units (SDU) have been completely received by the user equipment (UE).
As a first way, the eNodeB (i.e., an LTE-base station) can use the HARQ information (ACK/NACK status) to determine which HARQ processes have been successfully received by the user equipment. This information can then be mapped to successfully received RLC protocol data units (PDUs) and finally to RLC SDUs. However, the problem with this solution is that the HARQ feedback signaling is not fully reliable, which can lead to residual packet losses during the hand-over.
As a second way, it is possible to use RLC status report(s) to obtain reliable information on which RLC PDUs have been successfully received in the user equipment. The RLC status report contains an indication of the received and not received RLC PDUs. This information can then be mapped to the undelivered RLC SDUs. This method avoids the problem of residual packet losses, but requires the transmission of the RLC status report from the user equipment to the radio base station (eNodeB). This consumes uplink resources and leads to a delay before the forwarding can start.